Magnetoresistance elements are known to be manufactured in a variety of configurations, including, but not limited to giant magnetoresistance (GMR) elements, and anisotropic magnetoresistance (AMR) elements.
Referring to FIG. 1, a prior art GMR element 10 is formed having a plurality of layers, including an antiferromagnetic layer 12, a first pinned layer 14, a first non-magnetic layer 16, a second pinned layer 18, a second non-magnetic layer 20, and a free layer 22. In one conventional GMR element, the antiferromagnetic layer 12 comprises PtMn, the first and second pinned layers 14, 18 are comprised of CoFe, the first and second non-magnetic layers 16, 20 are comprised of a selected one of Ir and Ru, and the free layer 22 is comprised of NiFe. However, one of ordinary skill in the art will understand that other layers and materials can be provided in a GMR element.
The magnetoresistance element is used in a variety of applications, including, but not limited to current sensors responsive to an electrical current, proximity detectors responsive to proximity of a ferromagnetic object, for example, ferrous gear teeth, and magnetic field sensors responsive to a magnetic field external to the magnetic field sensor.
In each of the above applications, one or more magnetoresistance elements can be coupled either in a simple resistor divider or in a Wheatstone bridge arrangement. In either the resistor divider arrangement or in the Wheatstone bridge arrangement, one or more fixed resistors can also be used along with the one or more magnetoresistance elements. The resistor divider and the Wheatstone bridge arrangement each provide an output voltage signal proportional to a magnetic field experienced by the one or more magnetoresistance elements.
The magnetoresistance element has an electrical resistance that changes generally in proportion to a magnetic field in a direction of a maximum response axis of the magnetoresistance element. However, the electrical resistance changes not only in proportion to the magnetic field, but also in proportion to a temperature of the magnetoresistance element. The affect of temperature can be characterized as a temperature coefficient in units of resistance per degree temperature.
It will be recognized that the temperature coefficient of the magnetoresistance element, when used in a resistor divider or in a Wheatstone bridge arrangement, can adversely affect the expected output voltage signal of the resistor divider or the Wheatstone bridge. In particular, if the one or more resistors used in conjunction with the one or more magnetoresistance elements do not have the same temperature coefficient as the one or more magnetoresistance elements, then the output voltage signal of the resistor divider and the Wheatstone bridge arrangement will be responsive not only to a magnetic field, but also to temperature changes.
An open loop arrangement of a current sensor, a proximity detector, or a magnetic field sensor is a known circuit arrangement in which one or more magnetic field sensing elements are exposed to a magnetic field generated external to the circuit. A closed loop arrangement of a current sensor, a proximity detector, or a magnetic field sensor is a known circuit arrangement in which one or more magnetic field sensing elements are exposed to both a magnetic field generated external to the circuit and also to an opposing magnetic field generated by the circuit, so as to keep the resulting magnetic field in the vicinity of the one or more magnetic field sensing elements near zero. The closed loop arrangement has certain known advantages over the open loop arrangement, including, but not limited to, improved linearity. Conversely, the open loop arrangement has certain known advantages over the closed loop arrangement, including, but not limited to, improved response time.